The present invention relates in general to internal combustion engines of automotive vehicles and internal combustion engines for driving stationary equipment, and specifically to a gasoline-powered or natural gas-powered automotive internal combustion engine of the type using a catalytic converter in the exhaust system for exhaust cleaning purposes.
For example, at present most automotive vehicles are equipped with catalytic converters in the exhaust systems of the engines for converting the toxic air contaminants in the exhaust emissions into harmless compounds. In a closed loop fuel control system for the internal combustion engine, control means adjust the mass flow rates of air and fuel entering the engine to the desired air-to-fuel ratio, preferably in a narrow range approaching stoichiometry, in the following manner.
The engine operates on a fuel-air mixture. A sensor is mounted within the exhaust pipe upstream of the catalytic converter. Exhaust gases emitted from the engine pass through the exhaust pipe where the sensor is exposed to them. (The exhaust gases are then passed through the catalytic converter into a tailpipe for entry into the atmosphere.) Leads from the sensor extend to an electronic control unit. The electronic control unit is responsive to the output voltage of the sensor and generates a control signal to be sent via leads to the air-fuel mixture control means. The air-fuel mixture control means responds to the control signal of the electronic control unit and regulates the mass flow rates of air and fuel introduced into the engine as it is directed.
These systems typically use a "three-way" catalyst which is especially effective to oxidize the unburned combustible compounds such as hydrocarbons (HC) and carbon monoxide(CO) contained in the exhaust gases emitted from the engine cylinders. The "three-way" catalyst not only promotes oxidation of these combustible compounds but is operable to reduce nitric oxides (NO.sub.x) in the exhaust gases to N.sub.2 if the exhaust gases to be processed by the catalyst have a chemical composition within a certain range which is dictated by the air-fuel ratio of the mixture supplied to the engine cylinders. Thus, the catalytic converter using the "three-way" catalyst provides triple effects to process the three most important kinds of air contaminative compounds in the exhaust gases when the air-to-fuel mixture supplied to the engine cylinders is proportioned to an air-to-fuel ratio within a certain range.
Experiments have revealed that it is in a narrow range containing the stoichiometric air-to-fuel ratio of about 14.8 to 1 (for a gasoline powered engine) or 17.0 to 1 (for a natural gas powered engine) that enables the triple effect or "three way" catalytic converter to produce its maximum conversion efficiency against the three most common types of air contaminative compounds.
Thus, the closed loop or feedback mixture control system involves an exhaust sensor operative to detect the concentration of a prescribed type of chemical component, normally oxygen, contained in the exhaust gases emitted from the engine cylinders and produces a signal indicative of the detected concentration of the chemical component. Control of the oxygen content of the exhaust gases by control of the mass flow rates of air and fuel introduced into the engine, within a fairly narrow range for automotive internal combustion engines, will optimize the performance efficiency of the catalytic converter.
The temperature of the exhaust gases from the engine can vary according to the power output and speed of the engine, as is well known in the art. The temperature of the oxygen sensor exposed to those exhaust gases varies in the same way as the temperature of the exhaust gases vary. If the temperature of the oxygen sensor is lower than the minimum operable level, for example about 600.degree. F. for an electrochemical-type oxygen sensor, the oxygen sensor is unable to function and produce a signal accurately representing the oxygen content of the exhaust gases.
In the case where the exhaust gas temperature, and hence the oxygen sensor temperature, varies over a wide range, the accuracy of the sensor output is significantly affected because of its strong dependence on temperature. Even in situations in which the air-to-fuel ratio of the mixture is supplied to the engine cylinders at a constant set value, if the temperature of the exhaust gases, and hence the oxygen sensor temperature, is higher or lower than a predetermined level, the sensor output signal will deviate from the correct value. The deviation of the sensor output signal causes the control means to regulate the air-to-fuel ratio to a value different from the correct value. In such a situation, the total performances of the exhaust sensor and the closed loop control system are impaired and the effectiveness of the catalytic converter is reduced.
Thus, there has long been a need to minimize the temperature effects on the oxygen sensor of an internal combustion engine having a catalytic converter so that the only variable affecting the sensor is the oxygen content of the exhaust gases. If temperature effects on the oxygen sensor can be minimized, the signals from the oxygen sensor will accurately reflect the oxygen content of the fuel-air mixture, and the ratios of that mixture can be regulated for maximum efficiency of the catalytic converter.
It is therefore an object of the present invention to provide an improved system to control the air-to-fuel ratio of the mixture to be produced in the mixture supply system of an internal combustion engine of the type arranged with a catalytic converter so that the catalytic converter can produce at maximum efficiency against air contaminative compounds contained in exhaust gases from the engine cylinders.
It is another object of the present invention to provide a system in which the effects of variable exhaust gas temperatures on the sensor is reduced or effectively eliminated when the engine speed or load varies.
It is a further object of the present invention to provide a system that maintains the oxygen sensor at a reasonably constant temperature so that its output accurately represents the oxygen concentration in the exhaust gas.